1. Field of the Invention
The present invention relates to a semiconductor laser driving circuit in which rise and fall time of waveform when a pulse modulation signal is provided to a vertical cavity surface emitting laser is improved.
2. Description of the Related Art
Conventionally, a modulation electric current driving circuit in a semiconductor laser driving circuit for high-speed optical communication is constructed as an electric current switch type in which a transistor or a field effect transistor (FET) has a differential construction.
FIG. 1 shows an example of the conventional structure of a driving circuit of a vertical cavity surface emitting laser (referred to as xe2x80x9cVCSELxe2x80x9d hereinafter). Transistors Tr1 and Tr2 constitute a differential switch circuit, and an emitter of Tr1 and Tr2 is connected to a constant electric power source which supplies a constant electric current Im. A collector of Tr1 is connected to the cathode of the VCSEL by way of a resistor R1 and a voltage VCC is supplied to the anode of the VCSEL. On the other hand, a collector of Tr2 is connected to a resistor R2 and a voltage VCC is supplied to the other terminal of the resistor R2. A DC bias electric current supplying circuit is connected to the VCSEL on the cathode side thereof by way of an inductor L1, so that DC bias electric current Idc can be controlled.
Data is inputted from differential inputs TX+ and TXxe2x88x92 by way of an input buffer. When the logic xe2x80x9c1xe2x80x9d is inputted to the TX+, Tr1 is turned xe2x80x9cONxe2x80x9d and modulation electric current Im is supplied to the VCSEL, resulting in emission of light. When the logic xe2x80x9c0xe2x80x9d is inputted to the TX+, Tr1 is turned xe2x80x9cOFFxe2x80x9d and modulation electric current Im is not supplied to the VCSEL, resulting in a non-emission state of light.
In general, in a semiconductor laser driving circuit including a VCSEL, DC bias electric current which is approximately of the level of the threshold electric current Ith is constantly supplied and pulse modulation electric current is coupled thereto, in order to improve the turn-on delay of the laser.
However, in the case of a VCSEL, due to the carrier accumulation effect in its active region, its rise time and fall time tend to become long when the modulation is carried out at a high speed. Accordingly, there arises a problem that required rise and fall time and required specifications of the eye pattern mask may not be achieved in a VCSEL.
For example, in a modulation electric current driving circuit of the electric current switching type as shown in FIG. 1, since the modulation electric current is coupled with the DC bias electric current by DC coupling, the modulation electric current becomes zero when the light emitting element stops emitting light (at the time of the logic xe2x80x9c0xe2x80x9d level). However, as shown in FIG. 2A, the driving electric current does not decrease to the value of the DC bias electric current or lower. Therefore, the waveform of light output of the VCSEL has, as shown in FIG. 2B, quite large values of rise time tr and fall time tf due to the carrier accumulation effect to an active region of the VCSEL. Rise time and fall time are normally specified from 20% to 80% of the logic High level. In the case of a VCSEL, a portion larger than 80% or so rises up quite moderately in the rising and a portion smaller than 20% or so falls down quite moderately in the falling. Due to these moderate rise time and fall time, as shown in FIG. 3, the resulting eye pattern cannot suffice the predetermined eye mask of standard level.
The object of the present invention is to provide a semiconductor laser driving circuit in which rise and fall time of waveform when a pulse modulation signal is provided to a vertical cavity surface emitting laser is improved such that the eye pattern can suffice the predetermined eye mask of standard level.
In order to achieve the aforementioned object, there is provided a semiconductor laser driving circuit comprising: a bias electric current supplying circuit for supplying a bias electric current to a vertical cavity surface emitting laser; and a modulation signal circuit for amplifying input data according to frequency characteristics having a peak in the high value region thereof and coupling the amplified signal as a pulse modulation signal with the bias electric current.
Due to the structure described above, an overshoot and an undershoot can be added to the rise portion and the fall portion of the pulse modulation signal, respectively.
In a preferred embodiment of the present invention, the modulation signal circuit has a peaking adjustment circuit for variably adjusting peaking in the frequency characteristics.
In a preferred embodiment of the present invention, the modulation signal circuit has, at its output end, a capacitor for AC coupling.
In a preferred embodiment of the present invention, the modulation signal circuit has an attenuator on the downstream side of a voltage amplifier.
In a preferred embodiment of the present invention, the modulation signal circuit has a gain adjusting circuit for variably adjusting gains of a voltage amplifier.
Further, in order to achieve the aforementioned object, there is provided a semiconductor laser driving circuit comprising: a bias electric current supplying circuit for supplying a bias electric current to a vertical cavity surface emitting laser; and a modulation signal circuit for inputting input data and coupling the input signal as a pulse modulation signal with the bias electric current; and a differentiating circuit for inputting the input data, generating a differential waveform according to the input data and coupling the differential waveform with a rise portion and a fall portion of the pulse modulation signal.
Due to the structure described above, an overshoot and an undershoot can be added to the rise portion and the fall portion of the pulse modulation signal, respectively.
In the preferred embodiment of the present invention, the differentiating circuit has a buffer at its output end.
In the preferred embodiment of the present invention, the semiconductor laser driving circuit further comprises a variable attenuator provided on the upperstream side or on the downstream side of the differentiating circuit.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.